CSA arguing there own policies?

I'm a new poster but desperately need help. My DH has two children from a previous relationship, DD who he has no contact with who lives abroad and DS who he has on alterative weekends as well as his ex's other child who he brought up. We travel 120 miles every other weekend to pick up the kids and then take them home, which we will be continuing to do so, and take them out whenever we have them.

He has always paid CMS every month without fail even when he lost his job and my mother paid it for him to keep the peace with his ex.

I am a full time student in university and my DH has now caught the academic bug and is doing a full time accelerated course himself, so he can better himself, get a better paying job, and the children benefit from it in the long run as once he completes education he will be earning more and therefore they will receive more.

So my question is..
To my knowledge full time students are exempt from paying CMS as they are eligible for Nil Rate. Upon him speaking to the CMS today they have told him they can only process him for Nil Rate once he sends in his P45 from his job (which he resigned from to go to uni) which he still hasn't received and will continue to accrue arrears for CMS till they have received and processed this and has been told to continue to pay as normal even though hes now in full time education and we know we will be unable to claim any over payment back from his ex. They have also told him if he gets a part time job (as we will struggle financially with us both being students and as i work part time at 3 different jobs alongside full time university which isn't realistic for my own studies) his case adviser has told him, he will have to inform them for them to calculate how much he has to pay. I am very confused that this contradicts what it says on the CMS Gov.uk website in regards to students qualifying for Nil Rate.

We are thinking about going to Citizens Advice, but really not sure what to do, how are we supposed to deal with a organisation who dont even follow there own policies?

Are we correct he should no longer be paying as a full time student?

EDIT: he also applied for a variation around 6 months ago which they still have not processed so ultimately he could have been over paying for past six months also.